1. Field of the Invention
The present invention relates to a driving technique for an optical modulator used mainly for optical communication, and more particularly, relates to a driving method of an optical modulator, which enables long-distance transmission by suppressing transmission waveform deterioration due to a wavelength dispersion characteristic of a transmission path, and an optical transmitter and an optical transmission system using the driving method.
2. Description of the Related Art
In ultrahigh-speed and long-distance optical communications, waveform collapse of an optical signal becomes large due to a wavelength dispersion characteristic of an optical fiber. Therefore, a dispersion compensation fiber compensating for wavelength dispersion is necessary, in order to transmit the optical signal over a long distance. However, since the dispersion compensation fiber is expensive, a technique for transmitting ultrahigh-speed optical signals without dispersion compensation is required.
FIG. 24 is a diagram showing one example of an optical transmitter, to which a conventional pre-chirp technique is applied. This conventional optical transmitter sends out an optical signal pre-chirped by performing single-drive on an optical modulator formed by using, for example, a lithium niobate (LiNbO3: LN) substrate (hereinafter referred to as an LN modulator), to a transmission path. The pre-chirping is a technique for suppressing transmission waveform deterioration due to wavelength dispersion and a nonlinear effect, by giving a wavelength (frequency) variation within one pulse of a transmitted light in advance (refer to right side in FIG. 24). For example, when an α parameter representing a chirped amount given to the transmitted light is negative, and an optical fiber having a positive dispersion value with respect to a wavelength of the transmitted light is used for the transmission path, a pulse waveform of the optical signal propagated through the transmission path is compressed, and waveform expansion due to the wavelength dispersion at the transmission time is suppressed due to the waveform compression effect. As a result, the optical signal can be transmitted without deterioration of the waveform up to a certain distance (for example, to about 4 km for an optical signal of 40 Gb/s). However, if the transmission distance becomes longer than the above described distance (for example, 6 km), since a phase margin is lost, the optical signal cannot be transmitted. FIG. 25 illustrates the state of transmission waveform changes. The transmission waveform in FIG. 25 is one example of when an optical signal of 40 Gb/s and having the α parameter of −0.7 is transmitted using an optical fiber having a positive wavelength dispersion value.
As a conventional technique coping with the waveform deterioration in long-distance transmission as described above, for example, a technique has been proposed in which, in an optical receiver, a received optical signal is sampled by a monitor circuit using an A/D converter or the like to detect a deterioration amount in the received waveform, and a frequency characteristic of an equalizing amplifier is optimized corresponding to the waveform deterioration amount to perform dispersion compensation, thereby improving a reception characteristic (refer to Japanese Unexamined Patent Publication No. 2002-261692). Moreover, as a conventional technique relating to waveform shaping in the optical transmitter, for example, a semiconductor laser driving circuit for compensating for an oscillation delay of a semiconductor laser is known (refer to Japanese Unexamined Patent Publication No. 11-261485). In this semiconductor laser driving circuit, the oscillation delay of the semiconductor laser is compensated for by detecting continuous “0” in a data signal to change the pulse width of a driving waveform, to realize an optical transmission waveform of a desired shape.
However, the conventional technique has problems as described below. In the conventional technique for performing the dispersion compensation by the optical receiver, when dealing with an ultrahigh-speed optical signal such as 40 Gb/s, a high-speed operation is required for the A/D converter and the like. However, realization of an electric circuit capable of performing such a high-speed operation is not easy, and in the current technology, a large-scale control circuit such as a CPU is required, thereby causing a problem in that small size and low cost cannot be easily achieved. Moreover, since the frequency characteristic of the equalizing amplifier is optimized to perform the dispersion compensation, there is a disadvantage in that the phase margin cannot be improved as in the pre-chirp technique described above.
Even if a waveform shaped optical signal is transmitted from the optical transmitter by applying the conventional technique on the optical transmitter side as described above, the waveform of the optical signal after transmission becomes a collapsed waveform due to the wavelength dispersion characteristic of the transmission path, thereby making it difficult to extend the transmission distance.